Support both runners (ubi and alpine)

This change makes it possible to use either the UBI-based pipeline-runner or the alpine-based pipeline-runner

attached trigger runs (all successful):

• finished_pipelines-ubi.csv
• finished_pipelines-alpine.csv

changes:

oscap_scan.sh:

• alpine does not need to use podman to load a scanner for debian/suse, so it treats all oscap scans as native scans (exciting!)
• ubi9 uses python3.11 while alpine3.20 uses python3.12; oscap_scan.sh needs a common path to the python executable, so /usr/local/bin/pipeline-python is referenced
• similarly, site-packages dir (e.g. pip install) has a shell function to determine the correct path

manifest.sh:

• add the skopeo method for determining a multiarch manifest list sha, crane is no longer required (nor installed)